Field
This document relates to an active matrix type organic light emitting display, and more particularly, to an image quality compensation device and method for an organic light emitting display, which compensate for deterioration with the passage of driving time.
Related Art
An active matrix type organic light emitting display comprises a self-luminous organic light emitting diode (hereinafter, referred to as “OLED”), and has advantages such as fast response speed, high light emission efficiency, high luminance, and wide viewing angle.
The OLED, a self-luminous element, comprises an anode, a cathode, and an organic compound layer HIL, HTL, EML, ETL, and EIL formed between the anode and the cathode. The organic compound layers consists of a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. If a driving voltage is applied to the anode and the cathode, holes which have passed through the hole transport layer HTL and electrons which have passed through the electron transport layer ETL move to the emission layer EML and form excitons; and as a result, the emission layer EML generates visible light.
The organic light emitting display arranges pixels in a matrix form, each pixel comprising an OLED and a driving thin film transistor (TFT) controlling the driving current flowing through the OLED, and adjusts the luminance of the pixels according to the gray level of video data. The luminance of a pixel is proportional to the magnitude of the driving current flowing through the OLED, and this driving current is dependent on the electrical characteristics of the driving TFT.
Although it is preferable that the driving TFT's electrical characteristics such as threshold voltage, mobility, etc are the same for all pixels, they differ slightly from pixel to pixel due to many causes in reality. Differences in the driving TFT's electrical characteristics cause luminance differences between the pixels.
A variety of compensation methods for compensating for differences in the driving TFT's electrical characteristics are known. The compensation methods are categorized into internal compensation methods and external compensation methods. In an internal compensation method, threshold voltage differences between the driving TFTs are automatically compensated for within a pixel circuit. It is necessary that the driving current flowing through the OLED is determined regardless of the threshold voltage of the driving TFT for internal compensation, thus making the configuration of the pixel circuit rather complicated. Moreover, the internal compensation method is inappropriate to compensate for mobility differences between the driving TFTs.
In an outer compensation method, sensed voltages corresponding to the threshold voltage (or mobility) of the driving TFTs are measured, and an external circuit modulates video data based on these sensed voltages to compensate for threshold voltage (or mobility) differences. In other words, the external compensation method is to derive a compensation value for compensating for differences in electrical characteristics between the driving TFTs before product shipment, store it as initial compensation data in a nonvolatile memory, modulate input digital video data based on the initial compensation data stored in the memory at the time of normal driving after product shipment, and compensate for luminance differences caused by the differences in electrical characteristics between the driving TFTs.
This external compensation method cannot cope with deterioration with the passage of driving time because the organic light emitting display is driven only with the initial compensation data even after product shipment, and the organic light emitting display is therefore susceptible to afterimages. Accordingly, an improved external compensation method has been recently proposed to update compensation data by sensing driving TFT deterioration occurring after shipment again. However, this improved external compensation method also has the following problems.
First, the prior art improved external compensation method does not include a process of checking for errors in compensation data obtained by an update operation. If an abnormal power-off situation such as a blackout occurs during the update process, the compensation data to be stored in a memory is not normal. As such abnormal compensation data serves as a basis for subsequent compensation—modulated video data is applied to pixels in accordance with abnormal compensation data, deteriorated values of the pixels are sensed, with the modulated video data being applied to the pixels, and a new compensation value is derived from the sensed deteriorated values), the quality level of images in a subsequent driving operation is significantly decreased.
Second, the prior art improved external compensation method does not include an additional component for preventing abnormal data from being stored in a memory when compensation data is distorted due to the instability of driving power. The abnormal data decreases the accuracy of a subsequent compensation operation and the quality level of images.
This creates a demand for a new external compensation method which increases compensation capability.